applets/weather: Add missing null checks

Ismael Asensiorequested to merge
work/iasensio/weather-forecastview-amends into master

Fixes the QML warnings when modelData is undefined

ForecastView.qml:71: TypeError: Value is undefined and could not be converted to an object
ForecastView.qml:86: TypeError: Value is undefined and could not be converted to an object
ForecastView.qml:108: TypeError: Value is undefined and could not be converted to an object

Amends commit 79f77bda

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